Modems used in communication networks, such as traffic control systems, require a modulation circuit in their RF transmitter section that provides for the transmission of binary digital data (0 to about 100 kilobits per second) that are impressed on a carrier or center frequency signal in the megahertz range. Such modems may have, for example, 24 channels having actual center frequencies that vary from 30.05 MHz through 41.55 MHz each of which has a bandwidth of 100 kHz (i.e., .+-.50 kHz deviation), and each of which is separated from each other by 500 kHz. When used by a frequency shift keyed (FSK) transmission scheme, these center frequency signals are shifted in frequency in response to the FSK request composed of "mark" and "space" information. The mark information is commonly the intelligent part of the signal, wherein the on or 1 state is represented by a binary bit. Conversely, the space information is commonly represented by the non-presence (off or 0 state) of a binary bit. The modem, in response to the space signal shifts the carrier frequency associated with the mark signal by a predetermined frequency step, such as 100 kHz. A system that responds to the presence and absence of a FSK request is disclosed in U.S. Pat. No. 3,626,330 of Zalonis issued Dec. 7, 1971.
The modulation circuit may commonly have a single high frequency generator that includes an oscillator and which provides a very precise frequency signal for each of the channels of the modem. The use of a single precise frequency generator to provide the carrier frequency signal within an accurate bandwidth, and then shift such a carrier frequency by a discrete and accurate amount, presents certain difficulties. For example, the actual center frequency of the modem channels varying from 30.05 to 41.55 MHz represents a significant frequency span, that is, 11.5 MHz or over 38% change in frequency from channel 1 to channel 24. Using a frequency generator that varies over this relatively high range (38%) precludes the use of the same generator to provide a constant deviation bandwidth, that is 100 kHz, for each of the carrier frequencies of the modem channels. Further, the requirement of providing this large variance in the carrier frequency signals is not easily reconcilable with the requirement of shifting each of these carrier frequency signals, in response to an external signal, by a constant step in its frequency. It is desired that a single modulation circuit provide carrier frequencies that vary over a large range of the frequency spectrum, especially in the megahertz range, while at the same time provide a constant deviation bandwidth and a constant frequency shift when requested by an external source.
Accordingly, it is an object of the present invention to provide a modulation circuit for a modem that generates very precise carrier frequencies that are shifted by a constant frequency step, such as 100 kHz, in response to the signals of a frequency shift keyed (FSK) communication station, and maintain both the non-shifted and shifted carrier frequencies within a constant bandwidth, such as .+-.50 kHz, regardless of carrier frequency.
It is a further object of the present invention to provide such a modulation circuit that provides very precise frequency signals that are maintained within a desired bandwidth and which may be shifted, in response to an external signal, by a discrete frequency step in a substantially instantaneous manner so as to serve the needs of various communication networks.
It is a further object of the present invention to provide a circuit for controlling an oscillator that generates highly precise signals and allows such signals to be shifted by discrete frequency steps.